Mascart was born in Quarouble, Nord. Starting in 1858, he attended the École normale supérieure (rue d'Ulm), earning his agrégé-préparateur three years later. He acquired his doctoral degree in science in 1864. After serving at various posts in secondary education, in 1868 he moved to the Collège de France to become Régnault's assistant. Mascart was appointed to succeed Régnault as the tenured Régnault chair in 1872, which he held until his death. In 1878 he also became the first director of the Bureau Central Météorologique.
He was elected Perpetual Member (1884), Secretary, and in 1904 President, of the Académie des Sciences, and in 1892, Foreign Member of the British Royal Society. Mascart was elected vice president of the British Institution of Electrical Engineers in 1900, the first non-Briton to hold the post. He was also a grand officier of the Légion d'Honneur. Mascart founded Supélec in 1894.
Mascart's graduate student Henri Bénard carried out groundbreaking experiments in thermal convection, as part of his dissertation research, in Mascart's laboratory. Bénard's doctoral thesis was defended in 1901.
Mascart died in Paris at the age of 71. Obituaries were published in the Journal de Physique théorique et appliquée. and in Nature. Mascart's son-in-law Marcel Brillouin and his grandson Léon Brillouin were also noted scientists.
Cape Mascart is named for him.
- J. de Phys., 4th series, vol. 7, pp. 745-746 (1908).
- Nature", vol. 78, pp. 446-448 (1908).
- P. Janet (1909). La vie et les oeuvres de E. Mascart. Revue générales des sciences pures et appliquées vol. 20, pp. 574–593.
- E. Mascart (1866, 9th ed. in 1910). Éléments de Mécanique (Paris).
- E. Mascart (1876). Traité d'électricité statique (2 volumes, G. Masson, Paris).
- E. Mascart and J. Joubert (1883-1888). A Treatise on Electricity and Magnetism, Translated by E. Atkinson (2 volumes, T. De La Rue, London).
- E. Mascart (1894). The Age of Electricity. Report of the Board of Regents of the Smithsonian Institution, pp. 153–172.
- E. Mascart (1889-1893). Traité d'Optique (3 volumes, Gauthier-Villars, Paris).
- E. Mascart (1900). Traité de Magnétisme Terrestre (Paris).
- R.H. Stuewer (1981). Mascart, Éleuthère Élie Nicolas. Dictionary of Scientific Biography (Charles Scribner's Sons, New York), vol. 9, pp. 154–156.